In a typical radio frequency (“RF”) communication network, data and other signals are transmitted from one or more fixed base stations to one or more wireless units. Because most wireless units are carried between different locations, e.g., home, car, and work, the distance between a wireless unit and each of a network's base stations will typically vary over time. As the wireless unit moves away from one base station and towards another, because the wireless unit has a limited range, it may be necessary to transfer support of the wireless units between the two base stations. This process is referred to as “handoff.” Handoff occurs when primary control of a call has to be transferred from one base station to another as the user moves between cells. In a traditional, “hard” handoff, the connection to the current, “source” base station is broken, and then the connection to the new, “target” base station is made. If all the base stations in a CDMA-based network use the same frequency bandwidths, however, it is possible to make the connection to the new base station without breaking the connection to the current base station. (Each base station with a connection to the wireless unit is referred to as a “call leg” or “leg.”) This is known as “soft” handoff. Soft handoff requires less power, which reduces interference and increases capacity.
Because of their widespread, near ubiquitous use, wireless units are regularly used to initiate emergency services, e.g., calls to 911, direct calls to emergency service providers, or the like. Unlike landline telephones, however, where the location of the telephone unit is static (at least within the context of a designated address or other location), the locations of most wireless units are variable. Some wireless units are provided with location devices (e.g., GPS) for providing information to the network about the current location of the wireless unit, for use by emergency responders in locating the wireless unit in an emergency situation. For wireless units without such location devices, or for situations where the location device or system is malfunctioning, it is desirable for the network itself to determine the location of the wireless units.
One such method involves calculating the distance of a wireless unit from a base station based on the wireless unit's signal propagation delay (e.g., the time it takes for a signal to travel from the wireless unit to the base station), which is possible because the dependency between signal propagation time and distance is linear. Use of this method may require propagation delay values from different base stations operated by different network vendors, for triangulation purposes. Current communication standards, however, do not allow for the collation of this information at a source base station at the particular time when an emergency service request is initiated at a wireless unit. Instead, propagation delay information is automatically transmitted from a target base station of one vendor to a source base station of another vendor only in the following instances: (i) immediately following the target base station's acquisition of the wireless unit; and (ii) when the propagation delay measured at the target base station changes by more than a designated amount. In most cases, these instances will not correspond to when an emergency service request is initiated, leaving the source base station “blind” as to the propagation delay of the wireless unit at the target base station(s). This may result poor quality or malfunctioning emergency service communications in the wireless network.